To some extent, the running backwards is the job of the back four. It's the fact that nobody is running backwards (or running at all) that is the problem.
For that second Norwich goal the back four retreated to the box, in a line, forced the guy on the ball wide and stopped him at the edge of the box. So he turned inside, played the ball into an advancing midfielder who was stood totally on his own outside the box and walked straight through. He must have run past three of our midfielders to get there.
Arguably the back four did its job but the midfield cover was nowhere to be seen.